1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a micro controller unit (MCU) capable of increasing a data retention time.
2. Description of Related Art
Typically, electronic devices using batteries, such as, cellular phones, notebook computers, personal digital assistants (PDAs), and remote controllers, include memory devices, such as non-volatile memory devices or volatile memory devices, micro controller units (MCUs), etc.
Whether an MCU of an electronic device using a battery or batteries can operate or not may be determined according to a voltage level supplied by the battery/batteries. When the supplied voltage level decreases to a level less than or equal to a predetermined voltage level, the MCU cannot normally operate. Thus, the MCU included in the electronic device using the battery/batteries is likely to malfunction when the supplied voltage level deviates from the predetermined voltage level.
Thus, it is important to prevent the MCU included in the electronic device from malfunctioning according to changes of the supplied voltage level. To substantially prevent this malfunction, the MCU monitors whether the supplied voltage level becomes lower than a predetermined reference voltage level.
When the voltage level supplied to the MCU is lower than the predetermined reference voltage level, or becomes higher than the predetermined reference voltage level through battery change or charge, the MCU may perform a reset operation to prevent a malfunction before performing a normal operation. The electronic device using the battery/batteries includes low voltage detectors (LVDs) which detect the voltage level supplied from the battery/batteries.
FIG. 1 is a graph showing an operation state of an MCU depending on a battery voltage level. FIG. 2 is a flowchart of a method of operating an MCU according to a supplied battery voltage level. Referring to FIGS. 1 and 2, a first reference voltage level VLVD denotes a minimum voltage level by which the MCU can normally operate. A second reference voltage level VPOR denotes a minimum voltage level by which data can be stored in a data register (not shown) or a memory (not shown) included in the MCU.
Therefore, as shown in FIG. 1 when a voltage level Vb applied to the MCU is equal to or greater than the first reference voltage level VLVD, the MCU operates normally in an operation state A. When the voltage level Vb applied to the MCU is lower than the first reference voltage level VLVD and is higher than the second reference voltage level VPOR, a normal operation of the MCU is not guaranteed. Here, the MCU is in an operation state B.
When the voltage level Vb applied to the MCU is lower than the second reference voltage level VPOR, the MCU cannot normally operate. Here, the MCU is in an operation state C. When a battery is initially connected to the MCU or the MCU is initially charged, the MCU starts a reset operation, in block S105. The MCU initializes the data register and is prepared to normally operate, in block S110. The MCU normally operates in block S115 and enters into a stop mode in response to a stop command received from a CPU, in blocks S120 and S125.
In the stop mode, a clock for operating the MCU is stopped by the CPU, and an LVD is also turned off. When no external interrupts are applied to the MCU in the stop mode, the MCU keeps the stop mode, in block S130. When an external interrupt is applied to the MCU in the stop mode, the LVD is turned on, in blocks S130 and S135. The LVD checks whether the voltage level of the battery is higher than the first reference voltage level VLVD.
When the checked voltage level of the battery is higher than the first reference voltage level VLVD, the MCU performs a normal operation, and when the execution of the normal operation is completed, the MCU enters into the stop mode in response to the stop command in blocks S140 and S115.
When the external interrupt is applied to the MCU in the stop mode, and the voltage level Vb of the battery checked by the LVD is lower than the first reference voltage level VLVD, the MCU cannot normally operate and an operational error may be generated. This status is called a HALT status. In the HALT status of the MCU, the LVD maintains a turn on status, and checks the voltage level Vb received from the battery, in block S150.
When the checked voltage level Vb of the battery is lower than the first reference voltage level VLVD, the MCU maintains the HALT status in blocks S150 and S145. When the checked voltage level Vb of the battery becomes higher than the first reference voltage level VLVD by changing or charging the battery, the MCU performs a reset operation before performing a normal operation, in order to perform an errorless normal operation, in blocks S150 and S105.
When the voltage level Vb of the battery supplied to the MCU becomes lower than the second voltage level VPOR, data cannot be stored in the data register or the memory device included in the MCU, and thus an operation of the MCU cannot be guaranteed any more.
Hence, when the voltage level Vb of the battery becomes the voltage level VLVD by which the MCU can perform a normal operation by changing or charging the battery the MCU is initialized by performing the reset operation before performing the normal operation.
In the operation status B in which the MCU can store data but cannot normally operate, that is, when the voltage level Vb of the battery applied to the MCU is in between the first reference voltage level VLVD and the second reference voltage level VPOR, the MCU performs the reset operation before performing the normal operation. Although the conventional MCU in the operation status B can store data, the MCU is initialized by performing the reset operation in order to perform the normal operation. This initialization causes battery consumption and a reduction of the data retention time of the MCU.
Therefore, a need exists for an MCU capable of increasing a data retention time.